Track and hold architecture with tunable bandwidth

ABSTRACT

To date, bandwidth mismatch within time-interleaved (TI) analog-to-digital converters (ADCs) has been largely ignored because compensation for bandwidth mismatch is performed by digital post-processing, namely finite impulse response filters. However, the lag from digital post-processing is prohibitive in high speed systems, indicating a need for blind mismatch compensation. Even with blind bandwidth mismatch estimation, though, adjustment of the filter characteristics of track-and-hold (T/H) circuits within the TI ADCs can be difficult. Here, a T/H circuit architecture is provided that uses variations of the gate voltage of a sampling switch (which varies the “on” resistance of the sampling switch) to change the bandwidth of the T/H circuits so as to precisely match the bandwidths.

TECHNICAL FIELD

The invention relates generally to analog-to-digital converters (ADCs)and, more particularly, to time-interleaved (TI) ADCs.

BACKGROUND

Referring to FIG. 1 of the drawings, the reference numeral 100 generallydesignates a conventional ADC. ADC 100 generally comprises atrack-and-hold (T/H) circuit 102 and a sub-ADC 104 so that, inoperation, the ADC 100 can sample an analog input signal X(t) at aplurality of sampling instants and convert the sampled signal into adigital signal Y[n]. As is shown in FIG. 1, though, the T/H circuit 104generally comprises switches and capacitors. The switch has a non-zeroresistance, which causes the T/H circuit 102 to function as a filter(typically a single pole low pass filter).

Turning to FIG. 2, a model 200 of the ADC 100 is shown. In model 200,the filter aspects of the ADC 100 are represented by filter 202, whilethe remainder of the functionality of the ADC 100 is represented byideal ADC 204. Filter 202 has a transfer function in the time-domain ofh_(a)(t), which can, in turn, be represented in the frequency-domain as:

$\begin{matrix}{{H_{a}(\omega)} = {\frac{g_{a}^{\; \omega \; \Delta \; t}}{1 + {\left( \frac{\omega}{\omega_{a}} \right)}}.}} & (1)\end{matrix}$

where g_(a) is the gain of ADC 100, Δt_(a) is the time delay relative toa reference, and ω_(a) is the cutoff frequency (bandwidth). This model200 can be useful when determining mismatches for TI ADCs.

In FIG. 3A, an example of a TI ADC 300 can be seen. TI ADC 300 generallycomprises ADCs 100-1 to 100-M (where each of ADCs 100-1 to 100-Mgenerally has the same structure as ADC 100 from FIG. 1) that areclocked by divider 302 so that the outputs from ADCs 100-1 to 100-M canbe multiplexed by multiplexer 304 to produce digital signal Y[n]. Yet,when building TI ADC 300, ADCs 100-1 to 100-M are not identical to eachother; there are slight structural and operational variations. Theseslight variations result in Direct Current (DC) offset mismatches,timing skew, gain mismatches, and bandwidth mismatches between ADCs100-1 to 100-M.

Of the different types of mismatches listed, the performance impact, asthe result of bandwidth mismatches, are the weakest, and, to date, havelargely been ignored, but, in order to build a high accuracy (generallygreater than 6 bits), high speed (generally greater than 1 GS/s) TIADCs, bandwidth mismatches between interleaved ADC branches need to becorrected. Looking to TI ADC 300, the output spectrum when the inputsignal is a tone with frequency ω_(*) can be represented as follows:

$\begin{matrix}{{Y\left( ^{\; \omega} \right)} = {\sum\limits_{k = 0}^{M - 1}{\left( {\frac{1}{M}{\sum\limits_{a = 0}^{M - 1}{{H_{a}\left( \omega_{*} \right)}^{{- }\; \frac{2\pi \; k}{M}a}}}} \right){{\delta \left( {\omega - \omega_{*} - \frac{2\pi \; k}{M}} \right)}.}}}} & (2)\end{matrix}$

Assuming a 2-way TI ADC (M=2), which generally represents theupper-bound or worst-case for bandwidth mismatch, equation (2) can bereduced to:

$\begin{matrix}{{Y\left( ^{\; \omega} \right)} = {{\left( \frac{{H_{0}\left( \omega_{0} \right)} + {H_{1}\left( \omega_{0} \right)}}{2} \right){X\left( ^{\; \omega} \right)}} + {\left( \frac{{H_{0}\left( \omega_{0} \right)} - {H_{1}\left( \omega_{0} \right)}}{2} \right){X\left( ^{\; {({\omega - \pi})}} \right)}}}} & (3)\end{matrix}$

with a Spurious-Free Dynamic Range (SFDR) of

$\begin{matrix}{{S\; F\; D\; R} = {20{\log_{10}\left( \frac{{H_{0}\left( \omega_{0} \right)} + {H_{1}\left( \omega_{0} \right)}}{{H_{0}\left( \omega_{0} \right)} - {H_{1}\left( \omega_{0}\; \right)}} \right)}}} & (4)\end{matrix}$

The SFDR for an M-way interleaved TI ADC, therefore, can then bedetermined to be:

$\begin{matrix}{{{S\; F\; D\; R} = {\max\limits_{k}\left( {20{\log_{10}\left( \frac{A\lbrack 0\rbrack}{A\lbrack k\rbrack} \right)}} \right)}}{where}} & (5) \\{{A\lbrack k\rbrack} = {\sum\limits_{a = 9}^{M - 1}{{H_{a}\left( \omega_{0} \right)}^{{- }\; \frac{2\pi \; k}{M}a}}}} & (6)\end{matrix}$

Now, equation (1) can be applied to TI ADC 300 for the purposes ofsimulation so

$\begin{matrix}{{{H_{a}\left( \omega_{0} \right)} = \frac{1}{1 + {\; \tau_{a}\omega_{0}}}},{{{{for}\mspace{14mu} T_{S}} > \tau_{a}} = \frac{1}{\omega_{a}}},} & (7)\end{matrix}$

where T_(s) is the period of clock signal CLK. Such a simulation yieldsthat variations in bandwidth mismatches are dependent on gain mismatchesand timing skews and that (with high accuracy, high speed TI ADCs)bandwidth mismatch can significantly affect performance. An example of asimulation of the effect bandwidth mismatch can be seen in FIG. 3B fordifferent gain and skew compensations. Thus, to achieve the desired SFDR(i.e., greater than 70 dB) for a TI ADC, the bandwidths of ADCs withinthe TI ADC should be matched to be within 0.1% to 0.25%.

To date, however, no estimation algorithm or circuit exists to blindlydetermine bandwidth mismatches. The two most relevant conventionalcircuits, though, are described in the following: Satarzadeh et al.,“Bandwidth Mismatch Correction for a Two-Channel Time-Interleaved A/DConverter,” Proceedings of 2007 IEEE International Symposium on Circuitsand Systems, 2007; and Tsai et al., “Bandwidth Mismatch and ItsCorrection in Time-Interleaved Analog-to-Digital Converters,” IEEETransactions on Circuits and Systems II: Express Briefs, Vol. 53, No.10, pp. 1133-1137, Oct. 23, 2006. Neither of these circuits, though,adequately addresses blind bandwidth mismatch estimation.

Assuming, however, that one is able to adequately perform blindbandwidth mismatch estimation, adjustment of bandwidths of the T/Hcircuits (like T/H circuit 102) in TI ADC 300 can be difficult due atleast in part to the precision of the bandwidth matching. A switchedcapacitor arrangement included within the T/H circuit 102 would beundesirable because it would be difficult to implement, and capacitivetuning (such as with a varactor and a tuning voltage) would also beundesirable because of signal dependencies. Thus, there is a need for abandwidth adjustment circuit that can be adjusted from a blind bandwidthmismatch estimation.

Some other conventional circuits are: U.S. Pat. No. 5,500,612; U.S. Pat.No. 6,232,804; U.S. Pat. No. 6,255,865; U.S. Patent Pre-Grant Publ. No.2004/0070439; U.S. Patent Pre-Grant Publ. No. 2004/0239545; U.S. PatentPre-Grant Publ. No. 2009/0009219; and Abo et al. “A 1.5-V, 10-bit,14.3-MS/s CMOS Pipeline Analog-to-Digital Converter,” IEEE J. of SolidState Circuits, Vol. 34, No. 5, pp. 599-606, May 1999;

SUMMARY

A preferred embodiment of the present invention, accordingly, providesan apparatus. The apparatus comprises a clock divider that receives aclock signal; a plurality analog-to-digital converter (ADC) branchesthat each receive an analog input signal, wherein each ADC branchincludes: a delay circuit that is coupled to the clock divider; an ADChaving: a bootstrap circuit that is coupled to the delay circuit; asampling switch that is coupled to the bootstrap circuit; and acontroller that is coupled to the bootstrap circuit to provide a controlvoltage to the bootstrap circuit so as to control a gate voltage of thesampling switch to adjust the impedance of the sampling switch when thesampling switch is actuated; a sampling capacitor that is coupled to thesampling switch; and a correction circuit that is coupled to the ADC;and a mismatch estimation circuit that is coupled to each delay circuit,each correction circuit, and each controller, wherein the mismatchestimation circuit provides a control signal to each controller toadjust for relative bandwidth mismatches between the ADC branches.

In accordance with a preferred embodiment of the present invention, theapparatus further comprises a multiplexer that is coupled to each ADCbranch.

In accordance with a preferred embodiment of the present invention, thecorrection circuit adjusts the output of its ADC to correct for DCoffset and gain mismatch.

In accordance with a preferred embodiment of the present invention, thebootstrap circuit further comprises: a boost capacitor that is chargedduring a hold phase of the ADC; a transistor having first passiveelectrode, a second passive electrode, and a control electrode, whereinthe first passive electrode of the transistor is coupled to the boostcapacitor, and wherein the second passive electrode of the transistor iscoupled to the sampling switch; a passgate circuit that is coupled tothe delay circuit, that is coupled to the control electrode of thetransistor, and that receives the control voltage; and a skew circuitthat is coupled to sampling switch and that is controlled by the controlvoltage.

In accordance with a preferred embodiment of the present invention, thetransistor further comprises a first transistor, and wherein thepassgate circuit further comprises: a second transistor having a firstpassive electrode, a second passive electrode, and a control electrode,wherein the first passive electrode of the second transistor is coupledto the controller so as to receive the control voltage, and wherein thecontrol electrode of the second transistor is coupled to the delaycircuit, and wherein the second passive electrode of the secondtransistor is coupled to the control electrode of the first transistor;a third transistor having a first passive electrode, a second passiveelectrode, and a control electrode, wherein the first passive electrodeof the third transistor is coupled to the second passive electrode ofsecond transistor, and wherein the control electrode of the thirdtransistor is coupled to the delay circuit; and a fourth transistorhaving a first passive electrode, a second passive electrode, and acontrol electrode, wherein the first passive electrode of the fourthtransistor is coupled to the control electrode of the first transistor,and wherein the control electrode of the fourth transistor is coupled tothe sampling switch, and wherein the second passive electrode of thefourth transistor is coupled to the second passive electrode of thethird transistor.

In accordance with a preferred embodiment of the present invention, theskew circuit further comprises a fifth transistor having a first passiveelectrode, a second passive electrode, and a control electrode, whereinthe first passive electrode of the fifth transistor is coupled to thesampling switch, and wherein the control electrode of the fifthtransistor is coupled to the controller so as to receive the controlvoltage.

In accordance with a preferred embodiment of the present invention, thecontroller is a digital-to-analog converter (DAC).

In accordance with a preferred embodiment of the present invention, thecontroller is a charge pump.

In accordance with a preferred embodiment of the present invention, anapparatus comprising a clock divider that receives a clock signal; aplurality ADC branches that each receive an analog input signal, whereineach ADC branch includes: a delay circuit that is coupled to the clockdivider; an ADC having: a bootstrap circuit that is coupled to the delaycircuit; a sampling switch that is coupled to the bootstrap circuit; acontroller that is coupled to the bootstrap circuit to provide a controlvoltage to the bootstrap circuit so as to control a gate voltage of thesampling switch to adjust the impedance of the sampling switch when thesampling switch is actuated; a sampling capacitor that is coupled to thesampling switch; an output circuit that is coupled to the samplingcapacitor; and a sub-ADC that is coupled to the output circuit; and ancorrection circuit that is coupled to the ADC; a mismatch estimationcircuit that is coupled to each delay circuit, each correction circuit,and each controller, wherein the mismatch estimation circuit provides acontrol signal to each controller to adjust for relative bandwidthmismatches between the ADC branches; and a multiplexer that is coupledto each ADC branch.

In accordance with a preferred embodiment of the present invention, anapparatus is provided. The apparatus comprises a clock divider thatreceives a clock signal; a plurality ADC branches that each receive ananalog input signal, wherein each ADC branch includes: a delay circuitthat is coupled to the clock divider; an ADC having: a bootstrap circuitthat is coupled to the delay circuit; a PMOS transistor that is coupledto the bootstrap circuit; a controller that is coupled to the bootstrapcircuit to provide a control voltage to the bootstrap circuit so as tocontrol a gate voltage of the sampling switch to adjust the impedance ofthe sampling switch when the sampling switch is actuated; a samplingcapacitor that is coupled to the PMOS transistor at its drain; an outputcircuit that is coupled to the sampling capacitor; and a sub-ADC that iscoupled to the output circuit; and an correction circuit that is coupledto the ADC, wherein the correction circuit adjusts the output of its ADCto correct for DC offset and gain mismatch; a mismatch estimationcircuit that is coupled to each delay circuit, each correction circuit,and each controller, wherein the mismatch estimation circuit provides acontrol signal to each controller to adjust for relative bandwidthmismatches between the ADC branches; and a multiplexer that is coupledto each ADC branch.

In accordance with a preferred embodiment of the present invention, thePMOS transistor further comprises a first PMOS transistor, and whereinthe bootstrap circuit further comprises: a boost capacitor that ischarged during a hold phase of the ADC; a second PMOS transistor that iscoupled to the boost capacitor at its source and the gate of the firstPMOS switch at its drain; a passgate circuit that is coupled to thedelay circuit, that is coupled to the gate of the second PMOStransistor, and that receives the control voltage; and a skew circuitthat is coupled to sampling switch and that is controlled by the controlvoltage.

In accordance with a preferred embodiment of the present invention, thepassgate circuit further comprises: a third PMOS transistor that iscoupled to the controller at its source, the delay circuit at its gate,and the gate of the second PMOS transistor at its drain; a first NMOStransistor that is coupled to the drain of the third PMOS transistor atits drain and the delay circuit at its gate; and a second NMOStransistor that is coupled to the drain of the third PMOS transistor atits drain, the source of the first NMOS transistor at its source, andthe gate of the first PMOS transistor at its gate.

In accordance with a preferred embodiment of the present invention, theskew circuit further comprises a third NMOS transistor that is coupledto the gate of the first PMOS transistor at its drain and the controllerat its gate.

In accordance with a preferred embodiment of the present invention, thecontroller is a DAC or a charge pump.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiment disclosed may be readily utilized as a basisfor modifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a circuit diagram of a conventional ADC;

FIG. 2 is a block diagram of a model of the ADC of FIG. 1;

FIG. 3A is a circuit diagram of a convention TI ADC using the ADC ofFIG. 1;

FIG. 3B is an example of a simulation showing the effect of bandwidthmismatch on the Spurious-Free Dynamic Range (SFDR) of a TI ADC;

FIG. 4 is a circuit diagram of a TI ADC in accordance with a preferredembodiment of the present invention;

FIG. 5 is a circuit diagram of the T/H circuit of FIG. 4;

FIG. 6 is a circuit diagram of the bootstrap circuit of 5; and

FIG. 7 is a graph depicting the bandwidth for the T/H circuit of FIG. 5versus “on” resistance of the sampling switch of the T/H circuit of FIG.5.

DETAILED DESCRIPTION

Refer now to the drawings wherein depicted elements are, for the sake ofclarity, not necessarily shown to scale and wherein like or similarelements are designated by the same reference numeral through theseveral views.

Referring to FIG. 4 of the drawings, the reference numeral 400 generallydesignates a TI ADC in accordance with a preferred embodiment of thepresent invention. ADC 400 generally comprises ADC branches 402-1 to402-M, divider 404, multiplexer or mux 408, and a mismatch estimationcircuit 410. Each ADC branch 402-1 to 402-M also generally comprises(respectively) ADC 410-1 to 410-M, correction circuit 416-1 to 416-M,and adjustable delay element or circuit 418-1 to 418-M. Additionally,each ADC 410-1 to 410-M generally comprises (respectively) a T/H circuit410-1 to 410-M and a sub-ADC 414-1 to 414-M.

In operation, TI ADC 400 converts analog input signal X(t) to a digitalsignal Y[n]. To accomplish this, divider 402 divides a clock signal CLK(with a frequency of F_(S) or period of T_(S)) into M clock signals(each with a frequency of F_(S)/M) that are staggered by delay circuits418-1 to 418-M and provided to ADCs 410-1 to 410-M. This allows each ofADCs 410-1 to 410-M to convert the analog signal X(t) to digital signalsX₁(k) to X_(M)(k). The gain and DC offset adjustments are applied todigital signals X₁(k) to X_(M)(k) by correction circuits 416-1 to 416-Mto generate digital signals Y[1] to Y[M], which can then be multiplexedby mux 408 to generate a digital signal Y[N].

To generally ensure that signals Y[0] to Y[M−1] are matched, mismatchestimation circuit 410 calculates and compensates for gain mismatches,DC offset mismatches, timing skews, and bandwidth mismatches. Themismatch estimation circuit 410 is generally a digital signals processor(DSP) or dedicated hardware, which determines the gain mismatches, DCoffset mismatches, timing skews, and bandwidth mismatches and which canprovide adjustments for gain, DC offset, timing skew, and bandwidth tocorrection circuits 416-1 to 416-M and T/H circuits 412-1 to 412-M. Amore complete explanation of the mismatch estimation circuit 410 can befound in co-pending U.S. patent application Ser. No. 12/572,717, whichis entitled “BANDWIDTH MISMATCH ESTIMATION IN TIME-INTERLEAVEDANALOG-TO-DIGITAL CONVERTERS,” and which is incorporated by referencefor all purposes.

Turning now to FIG. 5, T/H circuits 412-1 to 412-M (hereinafter referredto as 412 for the sake of simplicity) can be seen in greater detail. T/Hcircuit 412 generally comprises a bootstrap circuit 502, a controller504, a sampling switch 51 (which is typically an NMOS transistor or NMOSswitch), a sampling capacitor CSAMPLE, and an output circuit 506. Inoperation, the bootstrap circuit 502 controls the actuation andde-actuation of the sampling switch 51 based at least in part on a clocksignal CLKIN (which is received from a respective delay circuit 418-1 to418-M) and a control voltage VCNTL from controller 504. Generally, themismatch estimation circuit 406 provides a control signal to thecontroller 504 (which may be a digital-to-analog converter (DAC) orcharge pump) to generate the control voltage VCNTL. The control voltageVCNTL, through the bootstrap circuit 502, is able to control the gatevoltage of the sampling switch S1 to adjust the impedance or “on”resistance of the sampling switch S1 when the sampling switch S1 isactuated.

Looking to FIG. 6, the bootstrap circuit 502 can be seen in greaterdetail. When the clock signal CLKIN is logic low (such as during a holdphase), inverter 508 turns transistor Q1 (which is typically an NMOStransistor) “on,” while passgate circuit (which generally comprisestransistors Q2, Q3, and Q5) maintains transistor Q4 (which is generallya PMOS transistor) in an “off” state. Assuming that signal CLKZ is logichigh so that transistors Q8 and Q9 (which are typically NMOStransistors) are in an “on” state and during this logic low period ofclock signal CLKIN, supply voltage VDD charges the boost capacitorCBOOST. When clock signal CLKIN transitions to logic high, passgatecircuit turns transistor Q4 “on,” while transistors Q1 is turned “off.”At this point, a voltage is applied to the gate of sampling switch S1 toturn it “on.” This gate voltage for sampling switch S1 is generated atleast in part from the discharge of capacitor CBOOST, the input signalIN (which is applied through transistor Q6), and the control voltageVCNTL (which is applied through the passgate circuit and the skewcircuit (which generally comprises transistors Q7 and Q8)). Generally,this control voltage VCNTL is applied to the source of transistor Q2(which is generally a PMOS transistor) and the gate of transistor Q7(which is generally an NMOS transistor) so as to adjust the gate voltageof sample switch S1. Thus, the gate voltage of the sampling switch S1can be easily controlled by varying control voltage VCNTL. Additionally,because the sampling switch S1 is generally a NMOS switch operating in alinear region, variation of this gate voltage varies the “on” resistanceof the sampling switch S1, which adjusts the filter characteristics (andbandwidth) of the filter created by the sampling switch S1, resistor R1,and sampling capacitor CSAMPLE.

To illustrate the operation to bootstrap circuit 502 and sampling switchS1, a graph depicting bandwidth of T/H circuit 412 versus “on”resistance for the sampling switch S1 can be seen in FIG. 7. As can beseen, the bandwidth for T/H circuit 502 varies between about 2.956 GHzat for a VCNTL DAC code of zero to about 3.051 GHz for a VCNTL DAC codeof 1023Ω. Thus, the bandwidths for multiple T/H circuits 412 (such as412-1 to 412-M) with nominal bandwidths of 3 GHz can be adjusted tomatch one another to between about 0.25% and about 0.1%.

Having thus described the present invention by reference to certain ofits preferred embodiments, it is noted that the embodiments disclosedare illustrative rather than limiting in nature and that a wide range ofvariations, modifications, changes, and substitutions are contemplatedin the foregoing disclosure and, in some instances, some features of thepresent invention may be employed without a corresponding use of theother features. Accordingly, it is appropriate that the appended claimsbe construed broadly and in a manner consistent with the scope of theinvention.

1. An apparatus comprising: a clock divider that receives a clocksignal; a plurality analog-to-digital converter (ADC) branches that eachreceive an analog input signal, wherein each ADC branch includes: adelay circuit that is coupled to the clock divider; an ADC having: abootstrap circuit that is coupled to the delay circuit; a samplingswitch that is coupled to the bootstrap circuit; and a controller thatis coupled to the bootstrap circuit to provide a control voltage to thebootstrap circuit so as to control a gate voltage of the sampling switchto adjust the impedance of the sampling switch when the sampling switchis actuated; a sampling capacitor that is coupled to the samplingswitch; and an correction circuit that is coupled to the ADC; and amismatch estimation circuit that is coupled to each delay circuit, eachcorrection circuit, and each controller, wherein the mismatch estimationcircuit provides a control signal to each controller to adjust forrelative bandwidth mismatches between the ADC branches.
 2. The apparatusof claim 1, wherein the apparatus further comprises a multiplexer thatis coupled to each ADC branch.
 3. The apparatus of claim 2, wherein thecorrection circuit adjusts the output of its ADC to correct for DCoffset and gain mismatch.
 4. The apparatus of claim 2, wherein thebootstrap circuit further comprises: a boost capacitor that is chargedduring a hold phase of the ADC; a transistor having first passiveelectrode, a second passive electrode, and a control electrode, whereinthe first passive electrode of the transistor is coupled to the boostcapacitor, and wherein the second passive electrode of the transistor iscoupled to the sampling switch; a passgate circuit that is coupled tothe delay circuit, that is coupled to the control electrode of thetransistor, and that receives the control voltage; and a skew circuitthat is coupled to sampling switch and that is controlled by the controlvoltage.
 5. The apparatus of claim 4, wherein the transistor furthercomprises a first transistor, and wherein the passgate circuit furthercomprises: a second transistor having a first passive electrode, asecond passive electrode, and a control electrode, wherein the firstpassive electrode of the second transistor is coupled to the controllerso as to receive the control voltage, and wherein the control electrodeof the second transistor is coupled to the delay circuit, and whereinthe second passive electrode of the second transistor is coupled to thecontrol electrode of the first transistor; a third transistor having afirst passive electrode, a second passive electrode, and a controlelectrode, wherein the first passive electrode of the third transistoris coupled to the second passive electrode of second transistor, andwherein the control electrode of the third transistor is coupled to thedelay circuit; and a fourth transistor having a first passive electrode,a second passive electrode, and a control electrode, wherein the firstpassive electrode of the fourth transistor is coupled to the controlelectrode of the first transistor, and wherein the control electrode ofthe fourth transistor is coupled to the sampling switch, and wherein thesecond passive electrode of the fourth transistor is coupled to thesecond passive electrode of the third transistor.
 6. The apparatus ofclaim 5, wherein the skew circuit further comprises a fifth transistorhaving a first passive electrode, a second passive electrode, and acontrol electrode, wherein the first passive electrode of the fifthtransistor is coupled to the sampling switch, and wherein the controlelectrode of the fifth transistor is coupled to the controller so as toreceive the control voltage.
 7. The apparatus of claim 6, wherein thecontroller is a digital-to-analog converter (DAC).
 8. The apparatus ofclaim 6, wherein the controller is a charge pump.
 9. An apparatuscomprising: a clock divider that receives a clock signal; a pluralityADC branches that each receive an analog input signal, wherein each ADCbranch includes: a delay circuit that is coupled to the clock divider;an ADC having: a bootstrap circuit that is coupled to the delay circuit;a sampling switch that is coupled to the bootstrap circuit; a controllerthat is coupled to the bootstrap circuit to provide a control voltage tothe bootstrap circuit so as to control a gate voltage of the samplingswitch to adjust the impedance of the sampling switch when the samplingswitch is actuated; a sampling capacitor that is coupled to the samplingswitch; an output circuit that is coupled to the sampling capacitor; anda sub-ADC that is coupled to the output circuit; and an correctioncircuit that is coupled to the ADC; a mismatch estimation circuit thatis coupled to each delay circuit, each correction circuit, and eachcontroller, wherein the mismatch estimation circuit provides a controlsignal to each controller to adjust for relative bandwidth mismatchesbetween the ADC branches; and a multiplexer that is coupled to each ADCbranch.
 10. The apparatus of claim 9, wherein the correction circuitadjusts the output of its ADC to correct for DC offset and gainmismatch.
 11. The apparatus of claim 9, wherein the bootstrap circuitfurther comprises: a boost capacitor that is charged during a hold phaseof the ADC; a transistor having first passive electrode, a secondpassive electrode, and a control electrode, wherein the first passiveelectrode of the transistor is coupled to the boost capacitor, andwherein the second passive electrode of the transistor is coupled to thesampling switch; a passgate circuit that is coupled to the delaycircuit, that is coupled to the control electrode of the transistor, andthat receives the control voltage; and a skew circuit that is coupled tosampling switch and that is controlled by the control voltage.
 12. Theapparatus of claim 11, wherein the transistor further comprises a firsttransistor, and wherein the passgate circuit further comprises: a secondtransistor having a first passive electrode, a second passive electrode,and a control electrode, wherein the first passive electrode of thesecond transistor is coupled to the controller so as to receive thecontrol voltage, and wherein the control electrode of the secondtransistor is coupled to the delay circuit, and wherein the secondpassive electrode of the second transistor is coupled to the controlelectrode of the first transistor; a third transistor having a firstpassive electrode, a second passive electrode, and a control electrode,wherein the first passive electrode of the third transistor is coupledto the second passive electrode of second transistor, and wherein thecontrol electrode of the third transistor is coupled to the delaycircuit; and a fourth transistor having a first passive electrode, asecond passive electrode, and a control electrode, wherein the firstpassive electrode of the fourth transistor is coupled to the controlelectrode of the first transistor, and wherein the control electrode ofthe fourth transistor is coupled to the sampling switch, and wherein thesecond passive electrode of the fourth transistor is coupled to thesecond passive electrode of the third transistor.
 13. The apparatus ofclaim 12, wherein the skew circuit further comprises a fifth transistorhaving a first passive electrode, a second passive electrode, and acontrol electrode, wherein the first passive electrode of the fifthtransistor is coupled to the sampling switch, and wherein the controlelectrode of the fifth transistor is coupled to the controller so as toreceive the control voltage.
 14. The apparatus of claim 13, wherein thecontroller is a DAC.
 15. The apparatus of claim 13, wherein thecontroller is a charge pump.
 16. An apparatus comprising: a clockdivider that receives a clock signal; a plurality ADC branches that eachreceive an analog input signal, wherein each ADC branch includes: adelay circuit that is coupled to the clock divider; an ADC having: abootstrap circuit that is coupled to the delay circuit; a PMOStransistor that is coupled to the bootstrap circuit; a controller thatis coupled to the bootstrap circuit to provide a control voltage to thebootstrap circuit so as to control a gate voltage of the sampling switchto adjust the impedance of the sampling switch when the sampling switchis actuated; a sampling capacitor that is coupled to the PMOS transistorat its drain; an output circuit that is coupled to the samplingcapacitor; and a sub-ADC that is coupled to the output circuit; and ancorrection circuit that is coupled to the ADC, wherein the correctioncircuit adjusts the output of its ADC to correct for DC offset and gainmismatch; a mismatch estimation circuit that is coupled to each delaycircuit, each correction circuit, and each controller, wherein themismatch estimation circuit provides a control signal to each controllerto adjust for relative bandwidth mismatches between the ADC branches;and a multiplexer that is coupled to each ADC branch.
 17. The apparatusof claim 16, wherein the PMOS transistor further comprises a first PMOStransistor, and wherein the bootstrap circuit further comprises: a boostcapacitor that is charged during a hold phase of the ADC; a second PMOStransistor that is coupled to the boost capacitor at its source and thegate of the first PMOS switch at its drain; a passgate circuit that iscoupled to the delay circuit, that is coupled to the gate of the secondPMOS transistor, and that receives the control voltage; and a skewcircuit that is coupled to sampling switch and that is controlled by thecontrol voltage.
 18. The apparatus of claim 17, wherein the passgatecircuit further comprises: a third PMOS transistor that is coupled tothe controller at its source, the delay circuit at its gate, and thegate of the second PMOS transistor at its drain; a first NMOS transistorthat is coupled to the drain of the third PMOS transistor at its drainand the delay circuit at its gate; and a second NMOS transistor that iscoupled to the drain of the third PMOS transistor at its drain, thesource of the first NMOS transistor at its source, and the gate of thefirst PMOS transistor at its gate.
 19. The apparatus of claim 18,wherein the skew circuit further comprises a third NMOS transistor thatis coupled to the gate of the first PMOS transistor at its drain and thecontroller at its gate.
 20. The apparatus of claim 19, wherein thecontroller is a DAC or a charge pump.